In the field of optical surveillance, there is always a need for improved detection, recognition and identification of a target at a distant range. Long distance surveillance usually implies the use of 2D focal plane arrays, also know as starring arrays or area image sensors, which have the advantage of recording a bi-dimensional image all at once, without the need to scan different parts sequentially. To obtain a high resolution with such arrays, it is however necessary to use long focal length and a low numerical aperture optics providing sufficient light collection for efficient imaging. In order to meet these two requirements, the resulting devices are generally large, bulky and heavy, making them costly to manufacture and awkward to handle.
One known solution to make an imaging system more compact is the use of a catadioptric design, which applies both reflective and refractive components in its construction. An example of such a design is Schmidt-Cassegrain objective. Cassegrain objectives are well known in the field of astronomy for the design of telescopes, and include two coaxial mirrors. The primary mirror has a concave optical surface reflecting the incoming light towards the secondary mirror, which generally has a convex reflecting surface focussing the light beam on a focal plane. The image formed is free of spherical aberration and is usually located at or behind the vertex of the primary mirror. In a Schmidt-Cassegrain variation, corrective lens elements are used either at the entrance of the objective or in the path of the light beam reflected by the two mirrors.
Referring to U.S. Pat. No. 6,593,561 (BACARELLA, et al) a basic design for a catadioptric objective is shown. An example of a more complex catadioptric imaging system combining reflective and refractive surfaces is shown in U.S. Pat. No. 6,366,399 (ROGERS). U.S. Pat. No. 5,729,376 (HALL et al) provides another such system where two imaging planes are provided.
It is also known in the art to improve the resolution of an image captured by a focal plane array by displacing the image over the surface of the array, detecting it at various positions and combining the resulting detected views, compensated for the displacement. The principle behind this approach is for example explained in section 5 of “European uncooled thermal imaging technology” by McEwen, SPIE vol 3061, pp 179-190. Various techniques are known in order to provide this image displacement. In U.S. Pat. Nos. 5,180,912 (MCEWEN et al) and 5,291,327 (MCEWEN), a refractive microscanning system based on a rotating refractive chopper has been suggested. U.S. Pat. No. 4,652,928 (ENDO et al) suggests the vibration of a CCD detector to oversample an incoming image. The displacement amplitude of the CCD has to be equal to half the pixel pitch of the detector. In U.S. Pat. No. 4,633,317 (UWIRA et al), a rotating refractive wedge provides a continuous circular displacement of an image over a detector, where the wedge type rotating element induces a lateral displacement of the image for a charge-coupled detector array. Alternatively an electromagnet based displacement of a primary large planar mirror, not intrinsically athermalized, is suggested. Finally, U.S. Pat. No. 5,798,875 (FORTIN et al) and U.S. Pat. No. 5,774,179 (CHEVRETTE et al) suggest a microscan system based on the lateral translation of a lens or alternatively of a set of lenses.
In spite of the above-mentioned technological advances, there is still a need for a compact and high-resolution imaging system which can be used for scene observation, and would provide adequate precision for various applications requiring detection, recognition and identification of a target or more generally surveillance.